Hollow pre-cast concrete members are commonly used in a variety of applications such as water conduits, culverts and the like. These members are typically manufactured by disposing a collapsible central core a predetermined distance from a removable outer jacket, pouring concrete between the core and jacket, allowing the concrete to cure, collapsing and removing the core, and removing the jacket.
For many years, the collapsible cores utilized in the pre-cast process have utilized a plurality of pin and pivot hinges disposed along the sides of the core that allow the core to collapse inward and be removed once the concrete had set. This design facilitates removal of the cores by allowing them to collapse. However, the use of pin and pivot hinges with collapsible cores has inherent problems.
First, it is common for concrete to accidentally spill into the hinges, causing the hinges to temporarily bind. This binding can be remedied by exerting a shock force about the hinges with a hammer to break up the concrete and cause the hinges to move, but such force should not be applied until the concrete has been fully set for fear of ruining the concrete member. Second, the hinges are subjected to a harsh environment in which concrete, sand, small rocks, and the like, are prone to infiltrate the hinges. This infiltration can also cause the hinges to bind, creating the same problems as described above in reference to hardened concrete. In addition, infiltration of particulates will, over time, cause the hinges to wear, introducing a degree of play into the side walls that hinders proper alignment and causes unwanted seams to appear on the insides of the concrete member. A number of United States patents address some of the problems inherent in the current methods, but each has its own inherent drawbacks.
U.S. Pat. No. 4,969,626 titled "Adjustable Form For Casting Concrete Culverts" describes a form for casting three sided culverts that utilizes what is described as "unique expandable, multiple pin configuration" in place of traditional pin and pivot hinges. The pin configuration described in this patent is a triple pivot hinge utlizing two pivot members attached at one end to the walls of the form and at the other to a pair or additional pivot members by a pair of pins. The additional hinge members are attached together by a third pin and are held together by a fastener assembly. At the mating areas between the walls and members are mating splines that provide sealing against the concrete and prevent lines from appearing on the inside of the concrete element. This design overcomes one of the drawbacks inherent in traditional single pin designs as the removal of hinges from the seam areas minimizes the effects of play created by worn hinges. However, concrete and other particulates may still enter the hinges, causing the hinges to bind, necessitating the breaking techniques described above, and to eventually wear out. Thus, the hinge design of the U.S. Pat. No. 4,969,626 patent does not allow the core to be removed before the concrete is fully cured and is still prone to failure over time.
U.S. Pat. No. 4,790,508 titled "Concrete Casting Form Including Retractable Core" describes a method an apparatus for casting hollow concrete structures with a core section that does not include hinges. This is accomplished by mounting a continuous resilient core member on a hydraulic ram, disposing the core inside of retractable outer walls, coating the walls of the core with a release agent such as oil, pouring concrete between the core and walls, curing the concrete, activating the hydraulic ram to retract the core, retracting the walls and removing the concrete structure from the mold. This method overcomes many of the drawbacks of the current system as it does not utilize hinges that can wear or bind and thus does not require full curing of the concrete and does not produce unwanted seams in the cured concrete structure. Despite this fact, the design of the U.S. Pat. No. 4,790,508 patent has a number of drawbacks. First, this design is limited to cores of specific dimensions and cannot be varied to allow different hollow dimensions to be obtained. Second, the use of a hydraulic ram is not practical for use at a job site and thus this system is limited to use at dedicated facilities. Third, the continuous resilient nature of the core members requires that a release compound, such as oil, be used to prevent adhesion of the concrete to the walls of the core. Thus, if oil is not spread evenly across the entire outside surface of the core, the core will have a tendency to adhere to the concrete and be difficult to remove.
Still another release method and apparatus are described in U.S. Pat. NO. 4,673,159 titled "Molding Forms For Making Precast Portal Assembly Sections For Culverts". In this patent, the inner sections of a three sided culvert form are held together by a linkage system that allows the inner section of the form to be released when a shaft is rotated in a predetermined manner. Like the pivot and pin hinge designs, this design is prone to binding and degradation by concrete and other particulates and thus has the same inherent drawbacks.
A collapsible core section for use in pre-casting hollow concrete elements that will not bind, is not susceptible to wear and failure, does not produce elements having parting lines, may be used to form hollows of differing dimensions, and does not require a uniform application of mold release compounds to facilitate removal is not known in the art.